Psilotum

Whisk ferns in the genus Psilotum lack true roots but are anchored by creeping rhizomes. The stems have many branches with paired enations, which look like small leaves but have no vascular tissue. Above these enations there are synangia formed by the fusion of three sporangia and which produce the spores. When mature, the synangia release yellow to whitish spores which develop into a gametophyte less than long. The gametophyte lives underground as a mycoheterotroph, tapping into mycorrhizal networks to access carbon and other nutrients. When the gametophyte is mature, it is monoicous, producing both egg and sperm cells. The sperm cells swim using several flagella and when they reach an egg cell, unite with it to form the young sporophyte. A mature sporophyte may grow to a height of or more but has no apparent The gametophyte of Psilotum is unusual in that it branches dichotomously, lives underground and possesses vascular tissue. The nutrition of the gametophyte appears to be myco-heterotrophic, assisted by endophytic fungi. File:Psilotum nudum Rhizome.jpg|Psilotum nudum rhizomes File:Psilotum nudum Sydney Opera House.JPG|Clump of Psilotum nudum plants File:Psilotum complanatum - Lyman Plant House, Smith College - DSC04275.JPG|Psilotum complanatum plant ==Taxonomy and naming==

The genus Psilotum was first formally described in 1801 by Olof Swartz and the description was published in Journal für die Botanik (Schrader). The name of the genus is from the Ancient Greek word psilos meaning "bare", "smooth" or "bald" referring to the lack of the usual plant organs, and the seeming lack of leaves. Subsequent classifications have maintained this placement. ==Species and distribution==

There are two species, Psilotum nudum and Psilotum complanatum, with a hybrid between them known, Psilotum × intermedium W. H. Wagner. The distribution of Psilotum is tropical and subtropical, in the New World, Asia, and the Pacific, with a few isolated populations in south-west Europe. The highest latitudes known are in South Carolina, Cádiz province in Spain, and southern Japan for P. nudum. In the U.S., P. nudum is found from Florida to Texas, and P. complanatum in Hawaii. ==Relation to ferns==

Psilotum superficially resembles certain extinct early vascular plants, such as the rhyniophytes and the trimerophyte genus Psilophyton. The unusual features of Psilotum that suggest an affinity with early vascular plants include dichotomously branching sporophytes, aerial stems arising from horizontal rhizomes, a simple vascular cylinder, homosporous and terminal eusporangia and a lack of roots. The sporangia of Psilotum are trilocular synangia resulting from the fusion of three adjacent sporangia, Molecular evidence strongly confirms that Psilotum is a fern (in the broad sense that includes horsetails) and that psilophytes are sister to ophioglossoid ferns. ==References==